The timber industry generates considerable quantities of wood residues. Bark is produced in the debarking of the logs. Sawing of the debarked logs produces slabs, edgings and sawdust. If the solid slab and edgings are chipped for export pulp chips, the chips must be screened and this produces additional residues in the form of undersized and oversized woodchips. The timber is often seasoned then dressed and docked to length before despatching to the markets. This produces dockings and planer shavings residues. In many circumstances, there is poor utilization of the wood residues. Some of the bark can fetch a market as a garden mulch if it is sized. Sawdust, chip fines and planer shavings are used by the mill for energy, in most cases, as a fuel for kiln drying timber and, in a small number of mills, for electricity generation. However, there are generally many more residues produced than the energy requirements of the sawmilling industry. Sawdust is also used for composting into garden potting mixes. Generally, there is a large surplus of residues as the markets for horticulture are diminishing as demand for the dark P.radiata bark has fallen as the lighter wood fibre product has gained in popularity. The use of chipped prunings is also increasing as Councils and householders are turning to recycling and self-sufficiency. Where the residues are not utilised, they must be disposed of by incineration and land-fill dumpimg. The disposal of wood residues not only puts a severe cost on the sawmilling operation, particularly with the pollution restrictions imposed on air quality, water effluent into water-ways and ground water and the diminishing land-fill availability, but it represents a loss of a potentially valuable wood resource. Unlike coal which can be left in the ground to mine at a later date, wood residues cannot be stored on a long term basis and need to be processed when produced. Wood residues are also bulky, as in the case of sawdust and shavings, and can also contain a considerable quantity of water. Processing or utilization in-situ offers the advantage of avoiding the high cost of transportation.
When wood is heated, it loses free and hygroscopic water after which it will carbonize at temperatures in excess of 270° C. Gas and vapours are produced during carbonisation which, at some stage, becomes exothermic. There are many complex reactions occurring at the same time in the thermal decomposition of the various chemical components of wood. Practical carbonizing temperatures are in the range of 400–700° C. in order to produce charcoal with low-volatile content without excessive shortening of equipment life.
The volatile products consist of combustible gases and vapors. The energy value of the volatile products represents some 50% of the gross calorific value of the original dry wood. Although there are significant proportions of valuable chemical compounds present in the volatile products, production on a larger scale is required to economically justify the fractionation and recovery of these compounds. However, the typical scale of operations in individual timber mills cannot produce economic quantities of volatile products. This material can present problems in handling due to its acidic, corrosive nature and it would be a serious pollutant if discharged into the environment. One way of handling the volatile products is to burn them as they are produced before they are able to condense. The waste heat can be recovered to supply the energy requirements of the industry, hence optimizing the thermal efficiency of the carbonization process.
A viable system for the sawmilling industry would perform the threefold purpose of disposal of the wood residues, supply the energy requirements of the milling and seasoning operations; and upgrade the excess material into a product which can provide a profitable return.
The applicant's earlier Australian Patent No. 547130 described a process that achieved the above aims. This earlier patent described a process for carbonizing wood by feeding wood into a fluidized sand bed preheated to a temperature above the carbonizing temperature. The fluidized bed was fluidized with a gas mixture that included an oxidizing gas. The reaction conditions within the bed was selected such that all or a major proportion of the volatile components of the wood were burnt during carbonization, either as the volatiles were produced in the bed or partially burnt in the bed and the remainder in an afterburner. The burning of the volatile components provided sufficient energy to supply the heat required by the process as well as provide an excess of heat. Charcoal produced by the process was recovered as product.
The process described in Australian Patent No. 547130 provides a satisfactory process for treating timber milling residues to obtain a value-added product.
The present inventors have now developed an improved process for carbonizing wood, such as timber milling residues.
Activated carbon is an amorphous form of carbon having a very high specific surface area. Activated carbon has high absorptivity for a large number of substances and is widely used as an adsorbent in many industries, including water treatment, sugar refining, gold mining, brewing, gas adsorption and air conditioning, to name but a few. Activated carbon may be obtained by the destructive distillation of wood, nut shells, animal bones or other carbonaceous materials. It is also possible to produce activated carbon by activating a carbon feedstock, such as charcoal. Activation occurs by heating the material to be activated to an elevated temperature, such as 800–900° C. with steam or carbon dioxide to produce a carbon material having high porosity and a specific surface area that may be in excess of 1000 m2/g.